(1) Field of the Invention
This invention relates to a radiographic apparatus for medical or industrial use and a radiation detection signal processing method, for obtaining radiographic images based on radiation detection signals outputted at predetermined sampling time intervals from a radiation detecting device as radiation is emitted to an object under examination. More particularly, the invention relates to a technique for eliminating time lags, due to the radiation detecting device, of the radiation detection signals taken from the radiation detecting device.
(2) Description of the Related Art
In a medical X-ray diagnostic apparatus which is a typical example of radiographic apparatus, a flat panel X-ray detector (hereinafter called “FPD” as appropriate) has recently been used as an X-ray detecting device for detecting X-ray penetration images of a patient resulting from X-ray emission from an X-ray tube. The FPD includes numerous semiconductor or other X-ray detecting elements arranged longitudinally and transversely on an X-ray detecting surface.
That is, the X-ray diagnostic apparatus is constructed to obtain an X-ray image corresponding to an X-ray penetration image of a patient for every period between sampling intervals, based on X-ray detection signals for one X-ray image taken at sampling time intervals from the FPD as the patient is irradiated with X rays from the X-ray tube. The use of the FPD is advantageous in terms of apparatus construction and image processing since the FPD is lighter and less prone to complicated detecting distortions than the image intensifier used heretofore.
However, the FPD has a drawback of causing time lags whose adverse influence appears in X-ray images. Specifically, when X-ray detection signals are taken from the FPD at short sampling time intervals, the remainder of a signal not picked up adds to a next X-ray detection signal as a lag-behind part. Thus, where X-ray detection signals for one image are taken from the FPD at 30 sampling intervals per second to create X-ray images for dynamic display, the lag-behind part appears as an after-image on a preceding screen to produce a double image. This results in an inconvenience such as blurring of dynamic images.
U.S. Pat. No. 5,249,123 discloses a proposal to solve the problem of the time lag caused by the FPD in acquiring computer tomographic images (CT images). This proposed technique employs a computation for eliminating a lag-behind part from each of radiation detection signals taken from an FPD at sampling time intervals Δt.
That is, in the above U.S. patent, a lag-behind part included in each of the radiation detection signals taken at the sampling time intervals is assumed due to an impulse response formed of a plurality of exponential functions, and the following equation is used to derive lag-free radiation detection signal xk with a lag-behind part removed from radiation detection signal yk:xk=[yk−Σn=1N{αn·[1−exp(Tn)]·exp(Tn)·Snk}]/Σn=1Nβn in which Tn=−Δt/τn, Snk=xk—1+exp(Tn)·Sn(k—1),and βn=αn·[1−exp(Tn)],where Δt: sampling intervals;
k: subscript representing a k-th point of time in a sampling time series;
N: the number of exponential functions with different time constants forming the impulse response;
n: subscript representing one of the exponential functions forming the impulse response;
αn: intensity of exponential function n; and
τn: attenuation time constant of exponential function n.
Inventors herein have tried the computation technique proposed in the above U.S. patent. However, the only result obtained is that the above technique cannot avoid artifacts due to the time lag and satisfactory X-ray images cannot be obtained. It has been confirmed that the time lag due to the FPD is not eliminated.
Then, Inventors have proposed a technique disclosed in Japanese Unexamined Patent Publication No. 2004-242741. This technique, in dealing with the time lags of the FPD, removes a lag-behind part due to an impulse response based on the following recursive equations a-c:Xk=Yk−Σn=1N{αn·[1−exp(Tn)]·exp(Tn)·Snk}  aTn=−Δt/τn   bSnk=Xk—1+exp(Tn)·Sn(k—1)   cwhere Δt: the sampling time interval;
k: a subscript representing a k-th point of time in a sampling time series;
Yk: a radiation detection signal taken at the k-th sampling time;
Xk: a corrected radiation detection signal with a lag-behind part removed from the signal Yk;
Xk−1: a signal Xk taken at a preceding point of time;
Sn(k−1): an Snk at a preceding point of time;
exp: an exponential function;
N: the number of exponential functions with different time constants forming the impulse response;
n: a subscript representing one of the exponential functions forming the impulse response;
αn: an intensity of exponential function n; and
τn: an attenuation time constant of exponential function n.
In the above recursive computation, coefficients of the impulse response of the FPD, N, αn and τn, are determined in advance. With the coefficients fixed, radiation detection signal Yk is applied to equations a-c, thereby obtaining a lag-free radiation detection signal Xk.
Initial values are set for the recursive computation as follows. A setting k=0 is made, and X0=0 in equation a and Sn0=0 in equation c are set as initial values before X-ray emission. Where the number of exponential functions is three (N=3), S10, S20 and S30 are all set to 0.
In determining initial values in this way, an assumption is made that no residual lag (i.e. lag signal value) due to a time lag exists in time of X-ray non-irradiation (k=0: first frame) which is the starting point of the recursive computation. This aspect will particularly be described hereinafter. FIG. 6 is a view showing a state of radiation incidence. FIG. 7 is a view showing a time delay corresponding to the radiation incidence of FIG. 6. FIG. 8 is a view showing a time lag situation where a lag (i.e. a lag-behind part) in radiography overlaps fluoroscopy. In the drawings, time t0-t1 represents incidence for radiography, and time t2-t3 incidence for fluoroscopy.
As shown in FIG. 6, when an incidence of X rays takes place during time t2-t3, lag-behind parts shown in hatching in FIG. 7 add to a normal signal corresponding to an incident dose. This results in a radiation detection signal Yk shown in thick lines in FIG. 7. The above technique disclosed in Japanese Unexamined Patent Publication No. 2004-242741 can remove the lag-behind parts, i.e. the hatched portions in FIG. 7, to obtain a proper signal.
In time of starting fluoroscopy, there is hardly any lag remaining from a preceding event as shown in FIG. 7. Thus, the above technique disclosed in Japanese Unexamined Patent Publication No. 2004-242741 has no problem in carrying out a correction for removing the lag-behind parts (called “lag correction”) with no problem.
However, such a lag characteristic is variable with sensors of the FPDs. With a sensor having large lags, a long-lasting lag resulting from a preceding process of fluoroscopy may overlap a next signal as an afterimage. When fluoroscopy is resumed immediately after radiography, as shown in FIG. 8, a lag of the radiography done during time t0-t1 overlaps the fluoroscopy (see k=0 in FIG. 8). In such a case, the prior technique disclosed in Japanese Unexamined Patent Publication No. 2004-242741 cannot cope with the problem. Then, sensors with a bad lag characteristic must be rejected as unacceptable.